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Vision Lucky
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- 2. History Extromissive theoryPlato--light flows from our eye Ptolemy Challenged later through experiments by Huygens, Newton Intromissive Theory Aristotle--object creates “material images” that enter our eyes
- 3. History Pointillist theoryof vision Abu Ali al-Hassan ibn al-Hasan ibn al-Haytham (Alhazen) Optical Scientist of Middle Ages We see a collection of points on surfaces of objects (1:1 ratio)
- 4. Optics Four basicbehaviors of light Reflection Refraction Diffraction Absorption
- 5. Reflection Light wavesbounce off objects
- 6. Diffraction The bendingof waves around small particles The spreading out of waves past small openings
- 7. Diffraction Huygens ThomasYoung “Double Slit” Experiment
- 8. Absorption Transfer ofenergy from light to object Frequency of light is at or near energy levels of electrons of matter Electrons absorb energy of light wave and change their energy state
- 9. Refraction Light bendsas it travels from one medium to another
- 10. Our Eyes Themost sensitive and delicate organ we have We are able to see the world and our brains receive the information from the world The images we see are made up of light reflected from the objects we look at
- 11. Components of EyeRetina Membranous, sensitive nerve tissue in the eye Converts images from the eye’s optical system into electrical impulse These impulses are sent along the optic nerve to the brain More than 125 Million Rods and 6 Million Cones
- 12. Parts of theEye Cornea Transparent portion of the outer eye Outward curvature Primary refractor of light to eye Cleaned and nourished from aqueous humor and tears Iris Muscular diaphragm Controls the size of the pupil Controls the amount of light that enters Colored portion
- 13. Parts of theEye A hole in the center of the iris Changes size in response to changes in lighting Larger in dim lighting conditions Smaller in brighter lighting conditions
- 14. Parts of theEye Macula- highly sensitive center of retina Detail Fovea A tiny spot in the center of the retina Very center of Macula Contains only cone cells Responsible for our sharpness of vision
- 15. Parts of theEye Optic Disk Small area of the retina where the optic nerve leaves the eye: any image falling here will not be seen Brain covers it up
- 16. Parts of theEye Choroid Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; also, because of the high melanocytes content, the choroid acts as a light-absorbing layer. Sclera tough, white outer covering of the eyeball; extraocular muscles attach here to move the eye
- 17. Parts of theEye Aqueous humor Clear watery fluid found in the anterior chamber of the eye; maintains pressure and nourishes the cornea and lens Vitreous humor Clear, jelly-like fluid found in the back portion of the eye: maintains shape of the eye and attaches to the retina
- 18. Parts of theEye Lens Transparent, biconvex structure Refracts light to be focused on the retina More spherical when focusing on close objects Flatter when focusing on faraway objects Along with the cornea, light rays are focused back together on the retina Forms image of object on the back of retina
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- 20. How We SeeLight Passes through Cornea Aqueous Humor Pupil Lens Vitreous Humor Retina-->Phtoreceptors Optic Nerve-->Brain Cornea refracts 70-80% Change from air to cornea is largest change in index of refraction Lens--20% Ciliary Muscles/Zonule Fibers
- 21. Focal Length Distanceto bring parallel rays into convergence Diopters --(1/f) measure of lens power
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- 23. How We See1:1 Light reflected from different points are converged in the eye Convex lenses--image forms upside down
- 24. Accommodation Lens changesshape to accomade vision from far and close distances Ciliary/Zonule Fibers Resting state: Lens fat At a distance--ciliary muslce relaxed, zonule fibers are tensioned->lens is pulled flat Close Distance--ciliary muscle is constricted, zonule fibers relaxed-->Lens rounds
- 25. Cells Five differentcells in retina Photoreceptors Bipolar Cells Ganglion Cells Horizontal Cells Amacrine cells
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- 27. Phototransduction Electrical changesin rods and cones cause electrical responses in other cells in the retina Lead to production of action potentials in neurons These neurons form optical nerve The place on the retina where the axons of neurons converge is called the blind spot
- 28. Phototransduction To convertlight energy to a change in membrane potential--G Protein coupled receptor Rhodopsin has molecule bound--photon of light releases Activates G-protein (Transducin) 2nd Messenger Cascade (Phosphodiesterase) Turns cGMP to GMP cGMP gated Na+ channels close Ion Channel closed Darkness--Rods are depolarized (release glu) Light hyperpolarizes
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- 30. Center/Surround Receptive FieldsCenter is from direct connections from photoreceptors Surround--mediated by horizontal cell connections on-center cell--stimulated when the center of receptive field exposed to light inhibited when the surround is exposed to light Off-center cell--opposite Emphasize contrast at borders (edges)
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- 32. Retinofugal Projection VisualPathway from Optic Nerve to Brain The optic nerves from the left and right eyes partially decussate in the optic chiasma Fibers from nasal retina cross over Leads to binocular vision Left visual field viewed through right hemisphere Overlap in visual fields-->Depth Travel through optic tracts to the Lateral Geniculate Nucleus (LGN) of the Thalamus
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- 34. Some Visual ProjectionsHypothalamus A small number of axons connect here control of sleep wake cycles Midbrain Pretectum controls pupillary light reflex, certain eye movements Superior Colliculus voluntary and involuntary eye movements Most go to LGN
- 35. Lateral Geniculate NucleusRelays vision to visual cortex Form optic radiations that terminate in the Primary Visual Cortex (Occiptal Lobe) Layered 1,2--magnocellular LGN layers, receive inputs from M-type Ganglion cells (color insensitive, respond transiently, center/surround) 3,4,5,6--parvocellular LGN layers, receive inputs from P-type ganglion cells (sustained response, center/surround, color sensitive) Intralayers ventral to numbered layers--koniocellular layers, inputs from non M/P-type ganglion cells Combines inputs from two eyes Receptive fields similar to retinal
- 36. LGN-Retinotopic Map Visualfield is mapped so two points adjacent in visual field and on retina are connected to two adjacent points on neural surface Retinotopic map is magnified relative to the fovea b/c more photoreceptors
- 37. Other Inputs 80%of inputs from primary visual cortex Brainstem Modulate responses of LGN neurons Ex. Responsiveness can be modulated by feeling
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- 39. Visual Cortex Alsoknown as Striate Cortex Area 17--primary visual cortex Layers I-Mostly Fibers II, III, IVA, IVB,V, VI--pyrimdal cells, outputs from cortex III IVB--other cortical areas V--Pons and superior colliculus VI--LGN IVCa and IVCb--stellate cells, receive inputs from LGN IVCa--magnocellular LGN pathway IVCb--parvocellular LGN pathway Vertical connections from IVC to III
- 40. Ocular Dominance ColumnsZebra stripes in layer IV (~0.5mm wide) Adjacent areas are right or left eye inputs Wiesel and Hubel Microelectrodes & cats
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- 42. Blobs Layers IIand II Cytochrome oxidase staining (mitochondrial activity) Blobs and ocular dominance columbs overlayed in layers II, III and IV Blobs and interblobs receive parvocellular LGN input (no color) Blobs also receive koniocellular input (color)
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- 44. Pathways Magnocellular Pathway(M-Channel) LGN to IVCa IVCa to IVB IVB cells have Orientation selectivity Direction selectivity for moving stimuli Binocular selectivity No Color sensitivity Specialized for analysis of object motion
- 45. Pathways Parvocellular InterblobPathway (P-IB) LGN to IVCb IVCB to layers II and II to interblobs Have Very selective to orientation No color sensitivity Binocular sensitivity Small rf Specialized for analysis of object shape
- 46. Pathways Koniocellular Pathway--BlobChannel LGN inputs koniocellular layers and parvocellular layers via layer IVCb Have: Monocular sensitivity Color No orientation selectivity Analysis of Object color
- 47. Synthesis Cortical Module--basicunit for processing visual stimuli in visual cortex Hubel and Wiesel-- 2mm X 2mm Cortical image of a point in space Two complete sets of ocular dominance columns (layer IV) Sixteen blobs (III) Two complete orientation columns(interblob layer III) Striate cortex has about 1000 that act in parallel
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